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Friday, July 18, 2008

Intro to Lubrication

Why have a section about lubrication in a magazine about GM Diesels?

Simply stated, there is a direct connection between lubrication and the health and life of your vehicles. GM has done its part by manufacturing these marvelous machines; each owner is solely responsible for their maintenance. As a Certified Lubrication Specialist (CLS), certified by the Society of Tribologist and Lubrication Engineers (STLE), I cringe at the misleading advertising, misinformation and lack of lubrication education for both consumers and service professionals. In this continuing series, Lube Notes, I seek to provide factual information on lubrication to allow readers to rise above all the advertising hype and half-truths in order to make informed decisions when selecting lubricants – engine oil, transmission fluid, gear lube – for their vehicles. With this goal in mind, I intend to educate, not indoctrinate. In fact, the information in these columns will equip you to see through the attempts at indoctrination that surround us.

I want to begin with some fundamentals of lubrication and, in subsequent editions of maxxTORQUE, progress through lubricant formulation and applications.

So come along for the ride and be sure to jot down questions as you read. You will always find my email address at the end of the article, and I will be glad to answer your questions. Also, I will select some of the questions to publish in the next issue of maxxTORQUE.

Fundamentals of Lubrication

When two surfaces slide or roll in contact with each other, friction is the force that resists that motion. It is important to remember that motion is required in order for friction to occur. There are several factors that affect the level of friction between the moving surfaces. I list these in no specific order or level of magnitude.

Surface finish: relative smoothness or roughness

Type of motion: rolling versus sliding

Load: amount of pressure pushing the surfaces together

Speed: rate of relative motion

Lubricant: base fluid, viscosity and additives

Temperature (affects the condition of the surfaces and the viscosity of the lubricating film)

Surface Furnish: Asperities

Viewing the surface of metal with a microscope reveals extremely small jagged peaks called asperities (Figure One). There are continuous peaks and valleys of metal almost like microscopic mountain rages. It is impossible to machine the metal surface fine enough to remove these asperities. Obviously, separating these asperities is paramount to reducing friction and resulting wear. Reviewing the factors listed that affect friction, it is apparent that lubrication is the factor most useful for limiting friction. The motion is set by application and the load and speed will be related to the work application. Temperature is usually a range determined again by the application. Now since lubrication is our vehicle for limiting friction and wear, it is important to identify the types, or regimes as they are called, of lubrication that are available and in which applications each regime comes into play.

A microscopic look at two metal surfaces as they contact one another: the white spots represent gaps in contact between the two objects. Where asperities contact each other, friction and wear occur.

Hydrodynamic Lubrication

Hydrodynamic (HD), or full fluid film, conditions occur where the fluid film of oil completely separates the moving surfaces preventing any contact between asperities (Figure Two). In this lubrication regime, friction between the surfaces is, for practical purposes, non-existent (having been prevented by the lubricant). Internal fluid friction exists but we will save that for another discussion. Under light loads, main bearings on the crankshaft of internal combustion engines will operate in this regime.

Hydrodynamic Lubrication occurs in conditions where the fluid film of oil completely separates the moving surfaces, preventing any contact between asperities.

Boundary Lubrication

The second regime, Boundary Lubrication refers to conditions where the oil film cannot prevent the asperities of lubricated surfaces, in relative motion, from coming into contact (Figure Three). As the asperities make contact, the metal literally spot-welds (adhesion) or is broken off (abrasion) causing wear of the surfaces. Friction is at maximum level and it is estimated that 70% of all wear results from boundary lubrication. Cylinder piston ring interface, camshaft lobes, tappets and connector rod bearings are some locations in the internal combustion engine where boundary lubrication exist a percentage of the time.

This graphic depicts the dynamic nature of friction and the lubrication that occurs. As components move, lubrication alternates between boundary and hydrodynamic regimes.

Elastohydrodynamic Lubrication

Elastohydrodynamic lubrication (EHL) refers to conditions where the moving surfaces have poor conformity and extreme pressures (Figure Four). Gears and rolling element bearings are the usual suspects for EHL. Oil under extreme pressure (being non-compressible) will begin to function like a solid (pseudo-solid) separating the asperities. The contact surfaces will respond elastically, actually depressing the metal. This elastic characteristic is the source of the Elastohydrodynamic lubrication terminology.

Understanding these three lubrication regimes will prepare you to distinguish how lubricants are specifically formulated to function optimally in a particular regime. For example, anti-wear additives are used for boundary lubrication while extreme pressure additives are used for elastohydrodynamic lubrication. Why the difference? Stay tuned... In succeeding articles, I will refer over and over to these basic regimes as I explain how and why lubricants are formulated for a specific task.

Elastohydrodynamic lubrication takes place in gears and rolling elements such as these ball bearings.